linux-kernel - Re: [PATCH v2] x86: Remove compat vdso support

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Message-ID: <CALCETrU+HNjsUf=QJEq9U4FhjqjdLq58f-AjSnwxWrfe2jOCxw@mail.gmail.com>
Date:	Tue, 11 Mar 2014 09:42:38 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stefani Seibold <stefani@...bold.net>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Dave Jones <davej@...hat.com>,
	Martin Runge <Martin.Runge@...de-schwarz.com>,
	Andreas Brief <Andreas.Brief@...de-schwarz.com>
Subject: Re: [PATCH v2] x86: Remove compat vdso support

On Tue, Mar 11, 2014 at 9:30 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Tue, Mar 11, 2014 at 9:14 AM, H. Peter Anvin <hpa@...ux.intel.com> wrote:
>>
>> As much as I wouldn't mind getting rid of the compat vdso, I really
>> don't understand why the trivial solution is being ruled out -- the
>> trivial solution being to just reserve a little more space in the fixmap
>> area.
>
> No, the trivial solution is to stop adding crap to it.
>
> And no, "just reserve a little more space for it" is neither trivial
> nor a good idea. The fixed VDSO address is very much at the top of the
> address space, so you can't allocate more space for it unless you do
> one of
>
>  (a) make it non-contiguous
>  (b) get rid of the hole that is the very last page
>  (c) mess with the vsyscall pages and make it contiguous "backwards"
>
> all of which sound like *horrible* ideas. Certainly not "trivial solution".

We can move it freely -- we just have to move it *once* when the
system boots.  There isn't even any real requirement for it to live in
the kernel range.

Is there an address where it's more or less guaranteed to be possible
to stick a vma?  The top of the non-randomized stack sounds like a
decent place, but I'm not really familiar with the address space
layout.

>
> No, the trivial solution is to not mess with that legacy page at all.
>
> Why is *that* trivial solution not on the table? Why the heck are
> people hell-bent on changing this stupid legacy page around?
>
> I find this whole thread very annoying. We shouldn't care about
> x86-32, and certainly not from a performance angle - we should
> consider it a "it's done, don't touch it" issue.

I'll let other people fight this particular battle :)

>
>                  Linus



-- 
Andy Lutomirski
AMA Capital Management, LLC
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